When manufacturing an article from ceramic material, the starting material can be a powder which is mixed with a plasticizer which constitutes a temporary binder during shaping of the powder into a body of a known technique for ceramic materials, for example by injection moulding. The temporary binder may consist, for example, of methyl cellulose, cellulose nitrate, an acrylic binder, a wax or a mixture of waxes. After the shaping of the powder body, the binder is driven off by heating, so that the powder body becomes substantially free of binder, and then the body is subjected to sintering. When it is a question of making an article of complicated shape using a material which is difficult to sinter, for example gas turbine blade wheels of silicon nitride, it is known to employ hot isostatic pressing for pressing the shaped powder body, since this method provides an all-sided pressure, uniform reduction of dimensions and insignificant distortion. In order to prevent pressurized gas from penetrating into the powder body during the hot isostatic pressing operation, the shaped powder body is surrounded by a gas-impermeable casing, usually of glass. Different methods of performing such hot isostatic pressing operations are described in greater detail in published British Patent Applications Nos. 2024255A (published Jan. 9th, 1980) and 2048952A (published Dec. 17th, 1980).
The driving off of the temporary binder requires a long time for powder bodies of large cross-section and there is a high risk of crack formation, because an internal overpressure may arise due to evaporation of the binder.
The present invention aims to provide an improved method of manufacturing an article of ceramic material of large cross-section, particularly an article having different portions of large and small cross-sections, for example a blade wheel for a gas turbine having a solid central body with projecting thin blades.